Combustion Apparatus And Applications Thereof

ABSTRACT

In one aspect, combustion apparatus are described herein which, in some embodiments, can provide advantageous NO x  loads while mitigating the formation of SCR catalyst deactivation species in the flue gas stream. In some embodiments, a combustion apparatus described herein comprises a furnace providing a flue gas stream, the furnace comprising a primary combustion zone having an air to fuel stoichiometric ratio ranging from 0.89 to 1.05 and air staging apparatus associated with the furnace.

RELATED APPLICATION DATA

The present application hereby claims priority pursuant to 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/512,398, filed Jul. 27, 2011, which is hereby incorporated by reference in its entirety.

FIELD

The present invention is related to the treatment of flue gas streams and, in particular, to the removal of NO_(x) from flue gas streams.

BACKGROUND

The removal of nitrogen oxides (NO_(x)) from flue gas streams is important in many industries, including power generation using coal fired power plants. A widely used method for removing NO_(x) from flue gas streams is selective catalytic reduction (SCR). Catalyst used in SCR processes facilitates the reduction of NO_(x) to nitrogen (N₂) and water (H₂O) by reaction with ammonia (NH₃). To reduce the NO_(x) load on SCR catalysts, air staging (or combustion staging) can be used. In a combustion apparatus using air staging, secondary air is diverted away from the initial or primary combustion zone and reintroduced into the combustion apparatus outside of the initial or primary combustion zone to create a secondary combustion zone. Combustion in the relatively fuel rich and air deficient initial combustion zone produces less NO_(x) than would be produced without air staging. Reducing the NO_(x) load on SCR catalyst can reduce the amount of catalyst needed to obtain a desired level of NO_(x) removal and can also reduce flue gas treatment costs by reducing the amount of NH₃ needed.

However, for combustion apparatus using lignite or subbituminous coal such as Powder River Basin (PRB) coal as a fuel, previous air staging methods can generate a fly ash that has a high SCR catalyst deactivation potential that can offset any advantages achieved by a reduced NO_(x) load.

SUMMARY

In one aspect, combustion apparatus are described herein which, in some embodiments, can provide advantageous NO_(x) loads while mitigating the formation of SCR catalyst deactivation species in the flue gas stream. In some embodiments, a combustion apparatus described herein comprises a furnace providing a flue gas stream, the furnace comprising a primary combustion zone having an air to fuel stoichiometric ratio ranging from 0.89 to 1.05, and air staging apparatus associated with the furnace. A catalytic reactor of the combustion apparatus receives the flue gas stream from the furnace, the catalytic reactor comprising catalyst for the selective catalytic reduction (SCR) of nitrogen oxides in the flue gas stream. The flue gas stream provided to the catalytic reactor from the furnace, in some embodiments, comprises a reduced amount of at least one SCR catalyst deactivation species in comparison to a flue gas stream provided from the furnace wherein the primary combustion zone of the furnace has an air to fuel stoichiometric ratio outside of 0.89 to 1.05. In some embodiments, the primary combustion zone of a combustion apparatus furnace described herein comprises an air to fuel stoichiometric ratio ranging from 0.9 to 0.99

For reference purposes herein, an air to fuel stoichiometric ratio of 1.0 indicates there is no excess air or oxygen present beyond the theoretical amount required for complete combustion of the fuel.

In another aspect, methods of decreasing catalytic deactivation of SCR catalyst are described herein. In some embodiments, a method of decreasing catalytic deactivation of SCR catalyst comprises reducing an amount of at least one catalytic deactivation species for the SCR catalyst in a flue gas stream from a combustion apparatus comprising a furnace and an air staging apparatus, wherein reducing the amount of the catalytic deactivation species comprises providing an air to fuel stoichiometric ratio ranging from 0.89 to 1.05 in the primary combustion zone of the furnace.

These and other embodiments are described in greater detail in the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an individual structural catalyst body of a catalytic reactor of a combustion apparatus according to one embodiment described herein.

FIG. 2 is a graph showing the amount of gas phase P₂O₅ in a flue gas stream versus primary combustion zone air to fuel stoichiometric ratio in accordance with some embodiments of combustion apparatus and methods of decreasing catalytic deactivation of SCR catalyst described herein. The values of the y-axis are in arbitrary units relative to a maximum amount of gas phase P₂O₅.

DETAILED DESCRIPTION

Embodiments described herein can be understood more readily by reference to the following detailed description, examples and drawings and their previous and following descriptions. Elements, apparatus and methods described herein, however, are not limited to the specific embodiments presented in the detailed description, examples and drawings. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations will be readily apparent to those of skill in the art without departing from the spirit and scope of the invention.

In addition, all ranges disclosed herein are to be understood to encompass any and all subranges subsumed therein. For example, a stated range of “1.0 to 10.0” should be considered to include any and all subranges beginning with a minimum value of 1.0 or more and ending with a maximum value of 10.0 or less, e.g., 1.0 to 5.3, or 4.7 to 10.0, or 3.6 to 7.9.

I. Combustion Apparatus

In one aspect, combustion apparatus are described herein which, in some embodiments, can provide advantageous NO loads while mitigating the formation of SCR catalyst deactivation species in the flue gas stream. In some embodiments, a combustion apparatus described herein comprises a furnace providing a flue gas stream, the furnace comprising a primary combustion zone having an air to fuel stoichiometric ratio ranging from 0.89 to 1.05 and air staging apparatus associated with the furnace. A catalytic reactor of the combustion apparatus receives the flue gas stream from the furnace, the catalytic reactor comprising catalyst for the selective catalytic reduction (SCR) of nitrogen oxides in the flue gas stream. The flue gas stream provided to the catalytic reactor from the furnace, in some embodiments, comprises a reduced amount of at least one SCR catalyst deactivation species in comparison to a reference flue gas stream provided from the furnace wherein the primary combustion zone of the furnace has an air to fuel stoichiometric ratio outside of 0.89 to 1.05.

The air staging apparatus can comprise any air staging or overfire air (OFA) apparatus not inconsistent with the objectives of the present invention. The air staging apparatus can comprise a sufficient number of ports and injection velocities to provide complete or substantially complete air coverage in the upper furnace to fully mix the fuel and air. Air staging apparatus or OFA systems are commercially available from a variety of sources including Babcock and Wilcox Company of Charlotte, N.C., Siemens AG of Munich Germany and STEAG Energy Services LLC of Kings Mountain, N.C.

In some embodiments of combustion apparatus described herein, the primary combustion zone can comprise various air to fuel stoichiometric ratios. The air to fuel stoichiometric ratio for the primary combustion zone, in some embodiments, can have any value provided in Table I.

TABLE I Air to Fuel Stoichiometric Ratio of Primary Combustion Zone Air to Fuel Stoichiometric Ratio 0.89-1.05 0.89-0.99 0.90-0.99 0.91-0.98 0.92-0.97 0.93-0.96 0.93-0.95 0.90-0.94 0.96-0.99

Combustion apparatus described herein, in some embodiments, further comprise a catalytic reactor for receiving the flue gas stream from the furnace/air staging apparatus, the catalytic reactor comprising catalyst for the selective catalytic reduction (SCR) of nitrogen oxides in the flue gas stream. The SCR catalyst can comprise any suitable catalyst not inconsistent with the objectives of the present invention. In some embodiments, the SCR catalyst comprises monolithic structural catalyst bodies. Monolithic structural catalyst bodies of combustion apparatus described herein can comprise an outer peripheral wall and a plurality of inner partition walls defining flow channels extending longitudinally through the catalyst bodies.

FIG. 1 illustrates a perspective view of an individual structural catalyst body of a catalytic reactor of a combustion apparatus according to one embodiment described herein. The structural catalyst body of FIG. 1 displays an outer peripheral wall (10) and a plurality of inner partition walls (11) within the outer peripheral wall (10). The inner partition walls (11) define a plurality of flow channels or cells (12) which extend longitudinally through the structural catalyst body. In some embodiments, the outer peripheral wall and inner partition walls are formed of a chemical composition comprising catalytically active metal functional group for the selective catalytic reduction of nitrogen oxides in the flue gas stream. Alternatively, in some embodiments, the outer peripheral wall and inner partition walls are formed of an inert or non-catalytic material suitable for supporting SCR catalytic material. In one embodiment, for example, the outer peripheral wall and inner partition walls are formed of cordierite.

In some embodiments, structural catalyst bodies of combustion apparatus described herein can comprise any of the monolithic structural catalyst bodies described in the following United States patents, each of which is hereby incorporated by reference in its entirety: U.S. Pat. No. 5,494,881, U.S. Pat. No. 7,776,786, U.S. Pat. No. 7,807,110, and/or U.S. Pat. No. 7,833,932.

In some embodiments, a catalytic reactor of a combustion apparatus described herein comprises one or more catalytic layers with each layer comprising a number of modularized sections. Each modularized section further comprises a metal support framework which holds an assembly of structural catalyst bodies in place wherein compressible packing materials between the catalyst bodies are used for proper flow distribution of fluid streams passing through the catalyst bodies.

In some embodiments of combustion apparatus described herein comprising structural catalyst bodies, one or more of the structural catalyst bodies can exhibit various catalytic activities after various time periods of use. For example, in some embodiments of combustion apparatus described herein comprising structural catalyst bodies, one or more of the structural catalyst bodies satisfies the equation y=(K/K_(o)), wherein K is the catalytic activity of the structural catalyst body after 16,000 hours of operation in the catalytic reactor, K_(o) is the initial catalytic activity of the structural catalyst body, and y ranges from about 0.6 to about 0.9. In other embodiments, y ranges from about 0.65 to about 0.85. In still other embodiments, y ranges from about 0.70 to about 0.80.

In some embodiments of combustion apparatus and methods described herein, providing a stoichiometric air to fuel ratio according to Table I in the primary combustion zone permits or assists in reduction of SCR catalyst deactivation species formed from combustion of the fuel, such a subbitumnous coal or lignite, thereby facilitating structural catalyst bodies to realize K/K_(o) values in the range of 0.6 to 0.9 after 16,000 hours of operation. In some embodiments of combustion apparatus described herein, the flue gas stream provided to the catalytic reactor from the furnace comprises a reduced amount of at least one SCR catalyst deactivation species in comparison to a reference flue gas stream provided from the furnace wherein the primary combustion zone of the furnace has an air to fuel ratio outside a range described in Table I.

A SCR catalyst deactivation species, in some embodiments, comprises a gas phase species. In some embodiments, a SCR catalyst deactivation species comprises a chemical catalytic deactivation species. A chemical catalytic deactivation species is operable to at least partially deactivate one or more SCR catalytic species by chemically binding to one or more active sites. A SCR catalyst deactivation species, in other embodiments, comprises a physical catalytic deactivation species, or a species that physically deactivates the SCR catalyst. In some embodiments, a physical catalytic deactivation species is operable to at least partially deactivate one or more SCR catalytic species by physically blocking access of flue gas species to catalyst active sites, such as by occluding catalyst pores and/or active sites. Catalytic deactivation species, in some embodiments, comprise one or more of P₂O₅, H₃PO₄, CaO, CaSO₄, SiO₂ and combinations or mixtures thereof. P₂O₅ and H₃PO₄, in some embodiments are chemical catalytic deactivation species by binding to active sites of SCR catalyst while CaO and CaSO₄ are physical catalytic deactivation species by occluding pore structure of the catalyst body.

In some embodiments, the flue gas stream provided to the catalytic reactor from the furnace comprises at least one SCR catalyst deactivation species in an amount at least 10 volume percent less than a reference flue gas stream provided from the furnace wherein the primary combustion zone of the furnace has an air to fuel ratio outside a range described in Table I. The flue gas stream provided to the catalytic reactor from the furnace, in some embodiments, comprises at least one SCR catalyst deactivation species in an amount at least 20 volume percent or at least 40 volume percent less than a reference flue gas stream provided from the furnace wherein the primary combustion zone of the furnace has an air to fuel ratio outside a range described in Table I. In some embodiments, operating the primary combustion zone with an air to fuel stoichiometry of Table I eliminates or substantially eliminates at least one SCR catalyst deactivation species from the flue gas stream in comparison with a reference flue gas stream provided from the furnace wherein the primary combustion zone of the furnace has an air to fuel ratio outside a range described in Table I. SCR catalyst deactivation species reduced in any of the foregoing amounts can comprises one or more of P₂O₅, H₃PO₄, CaO, CaSO₄ and/or SiO₂.

FIG. 2 is a graph showing the amount of gas phase P₂O₅ in a flue gas stream versus primary combustion zone air to fuel stoichiometric ratio in accordance with some embodiments of combustion apparatus and methods of decreasing catalytic deactivation of SCR catalyst described herein. The values on the y-axis are in arbitrary units relative to an arbitrary maximum amount of gas phase P₂O₅. As illustrated in FIG. 2, the catalytic deactivation species of P₂O₅ is substantially eliminated from the flue gas when operating the primary combustion zone to have an air to fuel stoichiometry ranging from 0.93-0.99.

In some embodiments of combustion apparatus described herein, the catalytic reactor has a catalyst potential. In some embodiments, catalyst potential is measured in units of (K/AV), where K is catalytic activity and AV is area velocity. In some embodiments, catalyst potential of a reactor of a combustion apparatus described herein can be measured against the catalyst potential of a catalytic reactor in communication with a furnace deficient of an air staging apparatus. In some embodiments, for example, a combustion apparatus described herein comprises a catalytic reactor having a catalyst potential (K/AV) of up to about 70% of the catalyst potential of a catalytic reactor in communication with a furnace deficient of an air staging apparatus. In some embodiments, the furnace deficient of the air staging apparatus is otherwise identical or substantially identical to the furnace of a combustion apparatus described herein.

In some embodiments, the furnace of a combustion apparatus described herein provides a flue gas stream to the catalytic reactor having a reduced nitrogen oxide load in comparison to a reference flue gas stream provided by an identical or substantially identical furnace not employing the air staging apparatus. In some embodiments, for example, the furnace of a combustion apparatus described herein provides a flue gas stream to the catalytic reactor having a nitrogen oxide load up to about 90% of a nitrogen oxide load of a reference flue gas stream produced by an identical or substantially identical furnace not employing the air staging apparatus and operating under identical or substantially identical conditions. In some embodiments, the furnace provides a flue gas stream to the catalytic reactor having a nitrogen oxide load up to about 85% or up to about 80% of a reference flue gas stream produced by a substantially identical furnace not employing the air staging apparatus. In some embodiments, the nitrogen oxide load of a reference flue gas produced by a substantially identical furnace not employing an air staging apparatus can be empirically and/or theoretically determined for purposes of comparison with a flue gas produced by a furnace of a combustion apparatus described herein.

Combustion apparatus described herein, in some embodiments, can be used with various fuels. Any suitable fuel not inconsistent with the objectives of the present invention may be used. In some embodiments, the fuel comprises subbituminous coal. In some embodiments, the subbituminous coal comprises Powder River Basin (PRB) coal. Moreover, in some embodiments, the fuel comprises lignite or mixtures of lignite and subbituminous coal. In some embodiments, a flue gas stream of a combustion apparatus described herein is provided by combustion of subbituminous coal, lignite or mixtures thereof in the furnace.

II. Methods of Decreasing SCR Deactivation

In another aspect, methods of decreasing deactivation of SCR catalyst are described herein. In some embodiments, a method of decreasing deactivation of SCR catalyst comprises reducing or lowering an amount of at least one catalytic deactivation species for the SCR catalyst in a flue gas stream from a combustion apparatus comprising a furnace and air staging apparatus, wherein reducing or lowering the amount of the at least one catalytic deactivation species comprises providing an air to fuel stoichiometric ratio ranging from 0.89 to 1.05 in the primary combustion zone of the furnace. In some embodiments, the air to fuel stoichiometric ratio in the primary combustion zone of the furnace is provided a value according to Table I hereinabove.

Further, in some embodiments, catalytic deactivation species decreased in the flue gas stream from the furnace can comprise any deactivation species described in Section I hereinabove. In some embodiments, for example, catalytic deactivation species are selected from the group consisting of P₂O₅, H₃PO₄, CaO, CaSO₄, SiO₂ and mixtures thereof.

In some embodiments, the flue gas stream provided to the catalytic reactor from the furnace comprises at least one SCR catalyst deactivation species in an amount at least 10 volume percent less than a reference flue gas stream provided from the furnace wherein the primary combustion zone of the furnace has an air to fuel stoichiometric ratio outside a range described in Table I. The flue gas stream provided to the catalytic reactor from the furnace, in some embodiments, comprises at least one SCR catalyst deactivation species in an amount at least 20 volume percent or at least 40 volume percent less than a reference flue gas stream provided from the furnace wherein the primary combustion zone of the furnace has an air to fuel stoichiometric ratio outside a range described in Table I. In some embodiments, operating the primary combustion zone with an air to fuel stoichiometry of Table I eliminates or substantially eliminates at least one SCR catalyst deactivation species from the flue gas stream in comparison with a reference flue gas stream provided from the furnace wherein the primary combustion zone of the furnace has an air to fuel stoichiometric ratio outside a range described in Table I.

In some embodiments of methods of decreasing catalytic deactivation described herein, the SCR catalyst comprises monolithic structural catalyst bodies. Monolithic structural catalyst bodies can have any construction and/or properties recited in Section I hereinabove. For example, in some embodiments, one or more of the monolithic structural catalyst bodies can comprise any of the monolithic structural catalyst bodies described in the following U.S. Pat. No. 5,494,881, U.S. Pat. No. 7,776,786, U.S. Pat. No. 7,807,110, and/or U.S. Pat. No. 7,833,932.

In some embodiments of methods described herein, the reduction of at least one catalytic deactivation species in the flue gas stream substantially results in one or more of the monolithic structural catalyst bodies satisfying the equation y=(K/K_(o)), wherein K is the catalytic activity of the structural catalyst body after 16,000 hours of operation in the catalytic reactor, K_(o) is the initial catalytic activity of the structural catalyst body and y ranges from about 0.6 to about 0.9. In some embodiments, y ranges from about 0.65 to about 0.85. In other embodiments, y ranges from about 0.70 to about 0.80.

In some embodiments of methods of decreasing catalytic deactivation described herein, the fuel combusted in the furnace to provide the flue gas stream comprises subbituminous coal. In some embodiments, the subbituminous coal comprises Powder River Basin (PRB) coal. The fuel combusted to provide the flue gas stream, in some embodiments, comprises lignite or a mixture of lignite with subbituminous coal.

Various embodiments of the invention have been described in fulfillment of the various objects of the invention. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the invention. 

That which is claimed is:
 1. A combustion apparatus comprising: a furnace providing a flue gas stream, the furnace comprising a primary combustion zone having an air to fuel stoichiometric ratio ranging from 0.89 to 1.05 and an air staging apparatus; and a catalytic reactor for receiving the flue gas stream from the furnace, the catalytic reactor comprising catalyst for the selective catalytic reduction (SCR) of nitrogen oxides in the flue gas stream.
 2. The combustion apparatus of claim 1, wherein the air to fuel stoichiometric ratio ranges from 0.90 to 0.99.
 3. The combustion apparatus of claim 1, wherein the air to fuel stoichiometric ratio ranges from 0.92 to 0.94.
 4. The combustion apparatus of claim 1, wherein the air to fuel stoichiometric ratio ranges from 0.96 to 0.99.
 5. The combustion apparatus of claim 1, wherein the air to fuel stoichiometric ratio ranges from 0.93 to 0.97.
 6. The combustion apparatus of claim 1, wherein the air to fuel stoichiometric ratio ranges from 0.93 to 0.95.
 7. The combustion apparatus of claim 1, wherein the SCR catalyst comprises monolithic structural catalyst bodies.
 8. The combustion apparatus of claim 7, wherein one or more of the structural catalyst bodies satisfies the equation y=(K/K_(o)), wherein K is the catalytic activity of the structural catalyst body after 16,000 hours of operation in the catalytic reactor, K_(o) is the initial catalytic activity of the structural catalyst body and y ranges from about 0.6 to about 0.9.
 9. The combustion apparatus of claim 8, wherein y ranges from about 0.65 to 0.85.
 10. The combustion apparatus of claim 8, wherein y ranges from about 0.70 to about 0.80.
 11. The combustion apparatus of claim 1, wherein the flue gas stream provided to the catalytic reactor by the furnace comprises a reduced amount of at least one SCR catalyst deactivation species in comparison to a reference flue gas stream provided from the furnace wherein the primary combustion zone has an air to fuel stoichiometric ratio outside of 0.89 to 1.05.
 12. The combustion apparatus of claim 1, wherein the flue gas provided to the catalytic reactor by the furnace comprises a reduced amount of at least one SCR catalyst deactivation species in comparison to a reference flue gas stream provided from the furnace wherein the primary combustion zone has an air to fuel stoichiometric ratio outside of 0.90 to 0.99.
 13. The combustion apparatus of claim 11, wherein the at least one catalyst deactivation species is selected from the group consisting of P₂O₅, H₃PO₄, CaO, CaSO₄, SiO₂ and mixtures thereof.
 14. The combustion apparatus of claim 13, wherein the flue gas provided to the catalytic reactor comprises the at least one catalyst deactivation species in an amount at least 10 volume percent less than the reference flue gas stream.
 15. The combustion apparatus of claim 13, wherein the flue gas provided to the catalytic reactor comprises the at least one catalyst deactivation species in an amount at least 20 volume percent less than the reference flue gas stream.
 16. The combustion apparatus of claim 1, wherein the flue gas stream provided to the catalytic reactor has a nitrogen oxide load up to about 90% of a nitrogen oxide load of a reference flue gas stream produced by a substantially identical furnace, operating under substantially identical conditions not employing the air staging apparatus.
 17. The combustion apparatus of claim 1, wherein the fuel comprises subbituminous coal.
 18. The combustion apparatus of claim 17, wherein the subbituminous coal comprises powder river basin (PRB) coal.
 19. The combustion apparatus of claim 1, wherein the fuel comprises lignite.
 20. A method of decreasing deactivation of selective catalytic reduction (SCR) catalyst comprising: reducing an amount of at least one catalytic deactivation species for the SCR catalyst in a flue gas stream from a furnace of a combustion apparatus comprising air staging apparatus, wherein reducing the amount of the catalytic deactivation species comprises providing an air to fuel stoichiometric ratio ranging from 0.89 to 1.05 in a primary combustion zone of the furnace.
 21. The method of claim 20, wherein the at least one catalytic deactivation species physically deactivates the SCR catalyst.
 22. The method of claim 20, wherein the at least one catalytic deactivation species chemically deactivates the SCR catalyst.
 23. The method of claim 20, wherein the at least one catalytic deactivation species is selected from the group consisting of P₂O₅, H₃PO₄, CaO, CaSO₄, SiO₂ and mixtures thereof.
 24. The method of claim 20, wherein the at least one catalytic deactivation species is H₃PO₄.
 25. The method of claim 20, wherein the air to fuel stoichiometric ratio ranges from 0.90 to 0.99.
 26. The method of claim 20, wherein the air to fuel stoichiometric ratio ranges from 0.93 to 0.97.
 27. The method of claim 20, wherein the flue gas stream comprises the catalytic deactivation species in an amount at least 10 volume percent less than a reference flue gas stream provided from the furnace wherein the primary combustion zone of the furnace has an air to fuel stoichiometric ratio outside of 0.89 to 1.05.
 28. The method of claim 20, wherein the flue gas stream comprises the catalytic deactivation species in an amount at least 10 volume percent less than a reference flue gas stream provided from the furnace wherein the primary combustion zone of the furnace has an air to fuel stoichiometric ratio outside of 0.93 to 0.97.
 29. The method of claim 20, wherein the flue gas stream has a nitrogen oxide load up to about 90% of a nitrogen oxide load of a reference flue gas stream produced by a substantially identical furnace operating under substantially identical conditions not employing the air staging apparatus.
 30. The method of claim 20, wherein the SCR catalyst comprises monolithic structural catalyst bodies.
 31. The method of claim 30, wherein the reduction of the at least one catalytic deactivation species substantially results in one or more of the monolithic structural catalyst bodies satisfying the equation y=(K/K_(o)), wherein K is the catalytic activity of the structural catalyst body after 16,000 hours of operation in the catalytic reactor, K_(o) is the initial catalytic activity of the structural catalyst body and y ranges from about 0.6 to about 0.9.
 32. The method of claim 31, wherein y ranges from about 0.65 to 0.85.
 33. The method of claim 31, wherein y ranges from about 0.70 to about 0.80.
 34. The method of claim 20, wherein the fuel comprises a subbituminous coal.
 35. The method of claim 34, wherein the subbituminous coal comprises Powder River Basin (PRB) coal.
 36. The method of claim 20, wherein the fuel comprises lignite. 